Modern immunosuppressive regimens based on immunosuppressive modalities such as calcineurin and mTOR inhibitors have improved survival and reduced morbidity in solid organ and bone marrow transplant patients. Current one-year solid organ graft survival rates are ˜90%. Acute rejection is no longer a major cause of organ failure and death; instead late organ failure due to chronic rejection has become the major hurdle impeding long-term graft survival.
Chronic allograft rejection, which manifests as progressive and irreversible damage to the graft from attack due to host immune responses, is the leading cause of graft failure after the first postoperative year. For example, 50% of lung and heart transplant patients have graft attrition at 5 and 10 years post-transplant, respectively, with roughly 50% of lung transplant patients having bronchiolitis obliterans syndrome (BOS) by year 5, and 50% of surviving cardiac transplant patients having chronic allograft vasculopathy (CAV) by year 8. Additionally, the frequency of chronic graft versus host disease (GVHD) in allogeneic bone marrow transplantation is roughly 50% by year 2. The duration of graft survival has remained essentially unchanged since cyclosporine was approved for use by the U.S. Food and Drug Administration in 1983, vividly demonstrating that chronic rejection continues unabated during treatment with available immunosuppressive inhibitors of calcineurin and mTOR.
Unlike acute rejection, chronic rejection is a gradual process of deterioration and failure that occurs later in the life of the transplant. The dramatic progression of cellular infiltration and allograft destruction seen with acute rejection is less pronounced in chronically rejected grafts. However, in contrast to treatable acute rejection, chronic rejection is conventionally irreversible when histologically detected, not preventable by any immunosuppressive regimen, and its pathogenesis is not conventionally understood. In solid organ transplants, chronic allograft rejection is characterized by progressive fibrosis and intimal proliferation rather than the acute inflammation and necrosis seen in acute rejection.
The immunopathology underlying chronic rejection has been investigated using small animal models. For example, there are mouse models for BOS based on heterotopic tracheal transplantation and bone marrow transplantation. An orthotopic/heterotopic trachea transplant model has elegantly shown that allogeneic airway epithelial cells are the primary target of the T cell response and, absent immunosuppression, both CD4 and CD8 T cells mediate rejection. A rat orthotopic lung allo-transplant model incorporating cyclosporine A (CsA) and rapamycin treatment reproduced the histopathology of BOS; similarly, rapamycin was ineffective in preventing CAV in a rat cardiac transplant model. The rodent model data are consistent with the clinical experience that current immunosuppressive drugs, including mTOR inhibitors that block IL-2 receptor signaling, do not effectively inhibit all of the T cell subset(s) mediating chronic rejection.
As previously noted, to date, the mechanism underlying chronic rejection has been poorly understood. To better understand why conventional immunosuppressive therapies that are so effective against acute rejection, but remain ineffective in preventing and/or treating chronic rejection, studies have focused on host immune response evoked by allograft transplants. In particular, intensive investigations in murine CAV models have provided some insight into an effector T cell subset mediating chronic allograft rejection. Primed CD8 T cells were sufficient to cause vasculopathy in completely MHC-mismatched aortic grafts in mice treated with cyclosporine. Intimal proliferation was independent of allo-MHC class I on the aortic graft, implying CD8 recognition of allo-MHC class II molecules. Similarly, in a nude mouse model, adoptive transfer of naïve CD8 T cells was sufficient to cause CAV in MHC class II-mismatched bm12 cardiac allografts; again implying CD8 recognition of allo-MHC class II molecules. In the latter study, CAV was dependent on IFN-γ, but not perforin or Fas ligand. An important study showed that CsA prevented vasculopathy caused by CD4 T cells, but was ineffective in preventing vasculopathy caused by CD8 T cells. Accordingly, at least in mice, CD8 T cells rather than CD4 T cells appear to have a calcineurin-independent pathway for T cell activation during chronic allograft rejection. Furthermore, conventional literature supports a central role for allogeneic epithelial cells as targets for chronic allograft rejection, and CD8 T cells making IFN-γ as effectors of chronic allograft rejection in the presence of calcineurin inhibitors.
Currently, chronic rejection is diagnosed by histopathological analysis, which typically requires an invasive biopsy of the allograft. These procedures are complex and often carry risks of bleeding, infection, or tissue perforation. Biopsy results may also be subject to interpretation and reproducibility issues due to sampling errors and inter-observer variabilities as the pathological criteria used to establish the diagnosis of chronic rejection (including the thickening of an intimal layer with luminal narrowing and fibrosis). Although less invasive imaging techniques have been developed for monitoring some forms of allograft rejection, these alternatives are also susceptible to limitations similar to those associated with biopsies.
Accordingly, a need exists for an accurate and easy-to-use tool for early prognosis and follow-up of chronic rejection, particularly for early prognosis of chronic rejection before any overt clinical or histological manifestation.